The present invention relates to a seat belt device having a retractor, and more particularly to a seat belt device equipped with a pretensioner for rotating a winding shaft of the retractor in a direction of removing a slack of the belt.
Conventionally, some retractors for the seat belt device can remove the slack of the belt (webbing) by means of the pretensioner, when the vehicle is in emergency such as collision. Also, other retractors have an energy absorbing mechanism for absorbing an impact on the body of the crew by letting out a predetermined amount of the belt, when a load acting on the belt exceeds a predetermined value after removing the slack of the belt by the pretensioner.
In JP-A-9-188223, a linear drive device (pretensioner) was disclosed, comprising a cylindrical tube, a piston having a hollow inner space and an end wall at one end portion in an axial direction to be slidable within the cylindrical tube, and a charge for subjecting the inner space of the piston to a high pressure gas after ignited, in which a passage opening (vent hole) is provided through the end wall and closed by a blow-by patch.
In this linear drive device, the piston is slid through the cylindrical tube by its high pressure gas when the charge is ignited, and the winding shaft of the retractor is rotated in a direction of removing the slack of the belt owing to a movement of the position. Thereafter, the winding shaft of the retractor is rotated reversely by activation of the energy absorbing mechanism. At this time, the piston is moved through the cylindrical tube against a pressure (residual pressure) of the high pressure gas in an opposite direction to the movement direction when the charge is ignited. That is, the piston further compresses the high pressure gas. Thus, the blow-by patch provided in the piston is broken or removed to exhaust the gas through the passage opening. Or the gas is exhausted through an exhaust opening provided in the blow-by patch.
In the case where there is no vent hole, the residual pressure is compressed by activation of the energy absorbing mechanism after the pretensioner is activated.
Therefore, an absorbed energy in the energy absorbing mechanism may be added to an energy for compressing the residual pressure of the pretensioner.
Furthermore, the pretensioner has generally a shorter activation stroke than the energy absorbing mechanism.
Hence, it is necessary to separate the pretensioner at a pretensioner stroke end, with no pressure added to the compression energy of the pretensioner, whereby the energy absorption amount has steps.
Since the residual pressure is further compressed, it is required to increase the pressure resistance of a pressure vessel.
To improve the above problems, a mechanism for releasing the blow-by patch at a predetermined pressure was disclosed in JP-A-9-188223. However, with this method, when the blow-by patch is released, the pressure is released literally, causing a sudden change.
In the case where the blow-by patch having a small hole is employed as disclosed in the above patent, a combustion gas can be vented gradually. However, if the blow-by patch is released, a sudden change occurs.
Also, if the hole diameter is greater not to release the blow-by patch, the venting is increased, producing a greater pressure loss when the pretensioner is activated.
Therefore, the design redundancy is reduced for making the pretensioner performance and the stability at the time of energy consumption consistent